In radiography it is important to have excellent image quality for the radiologist in order to make an accurate evaluation of a patient's condition. Important image quality aspects are image resolution and image signal-to-noise ratio (SNR).
In earlier technologies a combination in a screen/film arrangement of a prompt emitting luminescent phosphor screen together with a silver salt film was made, wherein the said film was made sensitive to ultraviolet, blue or green light, emitted by the luminescent phosphors after X-ray exposure through a patient.
A more recent technology, called “computed radiography” (CR) makes use, however, of absorption of captured X-rays and storage of the corresponding energy, transmitted through a patient, followed, in an electronic processing unit, by exciting the storage phosphor with energy (normally light in the red wavelength range emitted by a laser source) sufficient to release said stored energy by photostimulation by the said laser in form of visible light (normally in the blue wavelength range), wherein the released energy is read-out in a digital form, providing ability to reproduce and process an image and to enhance its diagnostic value. For this type of radiography, disclosed in basic patent U.S. Pat. No. 3,859,527 and called “computed radiography” (CR), the “signal-to-noise ratio” (SNR) depends on a number of factors.
The number of X-ray quanta absorbed by the storage phosphor screen is important therein as the SNR will be proportional to the square-root of the number of absorbed quanta. The so-called fluorescence noise, however, is of primary importance as well. This noise contribution originates from the fact that the number of photostimulated light (PSL) quanta detected for an absorbed X-ray quantum is small. Since much of the PSL is lost in the detection process in CR, fluorescence noise has an important contribution to the SNR. Hence, it is important that the number of photons detected per absorbed X-ray quantum is as high as possible. This situation is most critical e.g. in mammography, where X-ray quanta are used with low energy. Softer X-rays will give rise to less PSL centres and, therefore, to less PSL photons per absorbed X-ray quantum than harder X-rays. So in CR, a large number of PSL centres is created by an absorbed X-ray quantum. However, not all PSL centres are stimulated in the read-out process, because of the limited time available for pixel stimulation and because of the limited available laser power. Typically, only about 30% of the PSL centres is stimulated to give rise to a PSL photon. Since these photons are emitted and scattered in all directions, only 50% of the PSL photons escape from the storage phosphor screen at the detector side. Only a fraction of the PSL photons emitted at the top side of the storage phosphor screen is guided to the detector, which has a limited quantum efficiency itself. For that reason, the number of PSL photons detected per absorbed X-ray quantum is of the order of 1 to 5 and the fluorescence noise contribution is important in CR systems. In addition, it is well-known that fine detail visualisation, i.e. high-resolution high-contrast images are required for many X-ray medical imaging systems and, more particularly, in mammography. In phosphor screens, light scattering by the phosphor particles and their grain boundaries results in loss of spatial resolution and contrast in the image.
The number of PSL centres that is stimulated in the read-out process can be increased by reflecting the stimulating light at the bottom of the phosphor layer, i.e. by having a reflecting substrate. In this case the fraction of PSL centres that is stimulated will be higher than 30%. A reflecting substrate reflecting the PSL photons, thereby increasing the number that leaves the screen at the top side, provides a fraction to be higher than 50%. The combination of these effects may increase the number of PSL centres detected per absorbed X-ray quantum to a significant extent, thereby strongly improving the image SNR. However, when having a reflecting substrate, scattering is increased: the stimulating light spot is broadened when it is reflected at the screen substrate and spatial resolution is diminished. In powder CR screens, therefore, a reflective substrate is seldom used as such: it may, optionally, be used in combination with an anti-halation dye on top of it. The anti-halation dye, dedicated to absorb the stimulation light, thereby prevents its reflection and maintains high resolution. Anti-halation dyes however, although improving sharpness do not have the same influence on sensitivity of the CR plate, panel or screen.
Preparation steps in order to manufacture particularly useful screens or panels in favor of optimized speed increase combined with high definition (due to parallel aligned, vapor deposited phosphors in needle-shaped form) have been described in basic patent application WO 01/03156. In favor of image sharpness needle-shaped europium activated alkali metal halide phosphors, and more particularly, Eu-activated CsBr phosphor screens as described in US-Application 2001/007352 are preferred. In view of an improved sensitivity, annealing of said phosphors as in U.S. Pat. No. 6,730,243 is advantageously performed, said annealing step consisting of bringing the cooled deposited mixture as deposited on the substrate to a temperature between 80° C. and 220° C. and maintaining it at that temperature for between 10 minutes and 15 hours.
The high degree of crystallinity is easily analysed by X-ray diffraction techniques, providing a particular XRD-spectrum as has been illustrated in in US-Application 2001/007352. Therefore a mixture of CsBr and EuOBr or EuBr3 is provided as a raw material mixture in the crucibles, wherein a ratio between both raw materials normally is more than 90% by weight of the cheap CsBr and less than 10% of the expensive EuOBr, both expressed as weight %.
A europium activated cesium bromide phosphor giving an increased stimulated emission, and which is also suitable for use in the screen or panel, has, besides low amounts, homogeneously incorporated amounts of europium dopant, minor or neglectable amounts of trivalent europium versus divalent europium, which is measurable from emission intensities of divalent and trivalent europium ions present. Preferably said emission intensities are differing with a factor of at least 103, and more preferably even with a factor of 105 to 106 as has been set out in US-Application 2004/262535. Therein it has been shown that the more desired CsBr:Eu2+ phosphor in binderless storage phosphor panels or screens having such a needle-shaped columnar phosphor layer should have an amount of europium dopant versus CsBr in the range between 100 and 400 p.p.m., and even, more preferably, in the range between 100 and 200 p.p.m. as is measurable e.g. by means of X-ray fluorescence. In the further disclosure hereinafter p.p.m. will always be understood in terms of “molar amount ratio”. Incorporation of europium in minor amounts while making use of vapor deposition under reduced pressure and vacuum conditions has been shown therein to be favorable in order to reduce diffusion of the europium dopant and inhomogenous distribution of this main dopant, due to the quite severe heat vaporizing and depositing conditions. Said problem has also been treated in US-Application 2004/0104376, wherein it has been established that Eu as an activator element or dopant has properties that diffusion by heat is remarkable and that vapor pressure in a vacuum is high, so that there arises a problem that Eu is unevenly distributed in the main component because it is easily dispersed therein. Addition of Rubidium atoms to a photostimulable phosphor of the photostimulable phosphor layer so that a ratio of the Rb atoms to Cs atoms is 1/1,000,000 to 5/1,000 mol (corresponding with amounts in the range from 1 p.p.m. to 5,000 p.p.m.) is said therein to bring a solution in order to get high luminance, high sharpness and excellent durability. Much higher amounts of Rb versus Cs in the range from 150,000 to 2,000,000 p.p.m. were applied before as disclosed in U.S. Pat. No. 4,780,376.
On the other hand batches of raw materials may differ due to the presence of “impurities” like e.g. alkali metal salts such as sodium, potassium and/or rubidium salts, thereby giving rise in the end product to, at first sight, unexpected variations in speed. It is clear that there exists an ever lasting demand to further improve storage phosphor screens or panels from a point of view of a high and constant sensitivity and dopant homogeneity in order to provide ability to respond to the stringent demands with respect to high signal-to-noise ratio and definition of diagnostic images in computed radiography in general, and in mammography, in particular.